Modulation system



May 27, 1941.

N. H. CLOUGH MODULATION SYSTEM Filed May 6, 1939 2 Sheets-Sheet 1 11 4.5 [A v! m v2 w c y? INVENTOR NEWSOME HENRY LUUGH BY kg ATTORNEY 1941-N. H. CLOUGH 2,243,193

MODULATION SYSTEM Filed May 6, 1939 2 Sheets-Sheet 2 H 6 2 m w. r m w 2T 7 r E E o W n I A .0 S V m m Y N a I\ I 2 2 A V 2 A R m A E; c w 1 m.1= S H D H H l 1 T r C W 5 ll 1/ S S B V m T mm c m M v 1 A A R n K 1 WPatented May 27, 1941 warran- MODULATION SYSTEM Application May 6, 1939,Serial No. 272,676 In Great Britain April 22, 1938 7 Claims.

This invention relates to modulator circuit arrangements and has for itsobject to provide improved modulator circuit arrangements of higheficiency, comparative simplicity and cheapness, and comparativeimmunity from those known leakage resonance and similar disadvantageousefiects which occur in known high efficiency.

choke between said stages and four so-called class B modulator tubes soconnected as to constitute alternative valve paths from the D. C. sourceto each stage, two of the modulator tubes operating to secure modulationfor positive half waves and the other two operating to secure modulationfor negative half waves, the whole ar rangement being such that ineffect the high frequency stages are made to take power in parallel fromthe direct current source at the peaks of modulation.

The invention is illustrated in and further explained in connection withthe accompanying drawings.

In the drawings Fig. 1 illustrates somewhat diagrammatically mymodulation system wherein two class C stages are connected in series bya choke coil and the stages are per se shunted by modulator tubes witheach stage and the choke coil shunted by other modulator tubes, themodulator tubes operating class B and being modulated differentially inpairs to thereby modulate the potential on the high-frequency stagesand, consequently, their ability to convert direct current toalternating current power.

Figs. 2 to 5, inclusive, are modifications of the arrangement of Fig. 1.In Fig. 2, which is substantiallysimilar to Fig. l, the modulatingcircuits are shown and include means for applying more modulatingpotentials to a pair of the modulator tubes than to another pair. InFig. 3 a pair of the modulator tubes have their input electrodes excitedby the outputs of the other modulator tubes. In Fig. 4 the modulatingpotentials are applied in somewhat the same manner in which they willapply in Fig. 3. .In Fig. 4, however, radio-frequency chokes areinterposed between the high frequency stages and the modulationpotential choke. In Fig. 5 the modulating potentials are applied throughsub-modulators.

In order that the invention may be the better understood there willfirst be described with reference to Figure l a highly simplifiedcircuit which may be regarded as descriptive of the basic principle ofthe invention.

In the theoretical basic circuit of Figure 1 there is connected across asource HT of direct current potential, a series circuit consisting, inthe order stated, of a class C high frequency amplifier stage HFl, a lowfrequency choke CH, and a second class C high frequency amplifier stageHFZ. -The side of the stage HFI adjacent the positive terminal of thesource HT is marked A, the other side of said stage is marked B theremaining side of the choke CH is marked C and the side of the stage HF2adjacent the negative terminal of the source HT is marked D. A modulatortube VI has its anode connected to point Band its cathode connected topoint D and a second modulator tube V2 has its anode connected to pointA and its cathode to point C. The tubes VI and V2 operate in class B andare arranged to be conductive simultaneously on the same half cycle. Inorder to simplify Figure 1 the speech input circuits for the modulatortubes are not shown. The circuit constants are such that in the carriercondition the two high frequency stagesHFl I-IF2 contribute equally toa.

common output and the potentials between points high frequency stagesHF] and HF2, will besimultaneously increased. In the limit, when theinternal resistances of Vi and V2 become zero,

the whole voltage of the source HT will be applied across each highfrequency stage and the equivalent of peak-100% modulation will beobtained. In the basic circuit, as so far described, only the twomodulator tubes VI V2 are referred to, but two further correspondinglyconnected tubes Vi V2 are provided to deal with the other half waves ofmodulation the two said further tubes being connected between points Aand B and between points C and D respectively. It will usually be founddesirable to connect the negative end of the source, or some other pointthereon, or the centre point of the choke CH to earth.

In the above basic circuit it should be noted that as the four modulatortubes require the.

same grid-filament modulation input, two of the tubes, VI and V2 whosefilament-earth potential is not constant, being dependent on themodulationwill require different grid-earth input voltages from thoserequired by the other two tubes VI and V2 whose filaments are eitherearthed or at constant potential with relation to earth. A practicalcircuit in which this fact is taken into account is shown in Figure 2and comprises a direct current source HT, represented by two generatorsin series two high frequency stages HFI HFZ operated in class C and achoke CH all connected as in the basic circuit, and four modulator tubesoperated as before in class B and designated respectively VI, VI, V2,V2. Modulation input is applied by a transformer T having a secondary Swith a centre tap ST through which bias potential may, if required, beapplied or which may be connected directly to earth. One end of thetransformer secondary is connected, if required through a negative biassource as shown, to the grid of the tube VI and the other end of thetransformer secondary is connected, also through a bias source, ifrequired to the grid of the tube V2. Intermediate taps ITI, 1T2 on thetransformer secondary symmetrically disposed with relation to themid-point ST are connected, one, through a negative bias source ifrequired to the grid of VI and the other,

through a similar negative bias source if required to the grid of V22The anodes of VIand V2 are connected to the positive terminal of thesource HT and the cathodes of VI and V2 are connected at the negativeterminal thereof. The cathode of V2 is connected to the anode of V2 andto the point C. The cathode of VI' is connected to the anode of VI andto the point B.

It will be seen that with this circuit the grid of tube V2 must be movedin the positive direction by the amount by which the grid of tube VI hasbeen moved plus the amount by which the point C is moved, if the valveshave equal characteristics and identical bias settings and the gridtransformer taps are correctly disposed to achieve this result.

In the description as so far given, it has been assumed throughout thatthe points B and C at the ends of the choke are the only points ofconnection of the choke to the modulator tubes. This, however, is not anecessary feature and in order to ensure in a simple manner that thegridfilament potential of V2 shall move in the correct phase andamplitude relation with respect to that of VI with minimum difiicultydue to stray capacity efi'ects the circuit now to be described withreference to Figure 3, may with advantage be adopted. In this circuittwo class C high frequency stages I-IFI and HF2 and a choke CH areconnected as before across the direct current source HT. The choke CH,however, is provided with two intermediate taps B and C equidistant fromB and C respectively. The grid of VI is connected through a negativebias source to B, and the cathode of W is connected to B; the grid of V2is connected through a negative bias source to C and the cathode of V2to C'. The anode-cathode space of VI is connected between B and D andthat of V2 between C and D. Speech or other modulated input is appliedthrough a speech transformer T whose secondary S has its ends connectedrespectively to the grids of VI and V2, the mid-point of the secondarybeing connected through a negative bias source to the point D. In thisarrangement the tube V2 and, of course, also the tube VI is controlledautomatically by voltage set up in the choke CH. By properly choosingthe location of the points B C a modulation depth of 100% is obtainable.

In another arrangement shown in Figure 4 a series circuit consisting inthe order stated of a high frequency choke HFCI, a high frequency classC amplifier valve HFI, a second high frequency choke HFC2, a lowfrequency choke CH, a third high frequency choke HFCS, a fourth highfrequency choke HFC4, and a second high frequency class C amplifiervalve HFZ is connected between positive and negative terminals of asource HT of potential. The anode of the valve HFI is connected throughthe choke HFCI to the positive terminal of the source HT and the cathodeof the valve HF2 is connected direct to the negative terminal of thesaid source. In shunt across the series circuit consisting of the chokeI-lIF'CZ the choke CH and the choke HFC3 are two fixed condensers KI,K2. The anode of HFI is capacity coupled by condenser K3 to one end andthe anode of the tube HF2 is capacity coupled by condenser K4 to theother end of a parallel tuned circuit comprising an inductance L inparallel with the usual three plate condenser CP having in effect amid-tap. The mid-tap of the tuning condenser is connected to thejunction point between the two fixed condensers KI, K2, and the tunedcircuit inductance L is coupled to the output coil LI. The low frequencychoke CH is between the points B, C, and has a centre tap CHS which maybe connected to earth if required, and intermediate taps B C equidistantrespectively from B and C. The grid of a modulating tube VI isadjustably tapped upon a resistance RI which is in series with acondenser CI and a negative bias source GBI between the point B on theone hand and the cathode of the tube VI on the other, one plate of thecondenser CI being connected to B. Similarly the grid of a tube V2 isadjustably tapped upon a resistance R2 which is in series with acondenser C2 and a negative bias source GB2 between C and the cathode ofV2 one plate of the condenser C2 being connected to C. The tube VI hasits grid connected through a negative bias source to one end of thesecondary S of a speech input transformer T and tube V2 has its gridconnected through a negative bias source to the other end of saidsecondary. The said secondary has a centre tap ST which is connected tothe point D (this is the same as the negative terminal of the directcurrent potential source HT) through a negative bias source. The anodesof the tubes VI and V2 are connected to the point A (the positiveterminal of the direct current source HT); the cathodes of the tubes VIand V2 are connected to the point D; the anode of tube VI is connectedto the cathode of tube VI and to the point B; and the anode of the valveV2 is connected to the cathode of valve V2 and to the point C.

Figure 5 shows a further preferred modification in which, however,sub-modulator tubes VSI and VS2 are employed, the grid potentials (bothsteady and varying) for the modulator tubes VI and V2 being obtainedfrom across resistances ARI, AR2, in the anode circuits of thesub-modulators VSI VS2 respectively. The grids of valves VSI VS2 aredriven in phase with, but usually carry smaller amplitudes than thegrids of VI and V2 respectively, 1. e. the grids of VSI VS2 are drivenin phase opposition. The drive may be obtained in any of a variety ofways. For exam ple (this is not shown) speech input may be effected bymeans of a transformer having two push-pull secondaries, oneconnectedbetween theridsjof'VJ andVZ' andthe se'cond'connected betweenlthe' gridsof 'VSI andVSZ; the i centre points of the two push-pull secondariesbeing tiometers Pl P2 connected as shown, may be provided for adjustingthe relative amplitude of the speech inputs to V5! and VS2 on the onehand and to VIVZ' on the other, and speech input to VSIVS2VIV2' beeffected from sub-sub-modulator tubes VSSIVSS2 (which are fed from thespeech input transformer T) by resistance-capacity coupling effected byelements KRAI, KRAZ, and KCIKCZ.

In certain of the figures batteries are shown as providing anode andgrid bias voltages. For example in Figure 5 a battery is represented assupplying anode voltage to the sub-sub-modulators (VSSI and VSS2) andgrid bias to these tubes and to the sub-modulators (VSI and VSZ).Obviously any other suitable potential sources may be used with orwithout smoothing filters and/or potentiometer adjustment means as maybe required.

It is not absolutely necessary that the outputs of the two highfrequency stages be combined in a single circuit. For example, in a casewhere the same programme is required to be transmitted on two differentwave lengths simultaneously, the two high frequency tubes may operate ondifferent wave lengths with individual high frequency input drives and,of course, individual output circuits.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed I declare thatwhat I claim is:

1. In a modulation system, two class C high frequency amplifiers, asource of direct current potential, a modulation potential choke coil, acircuit connecting said high frequency amplifiers and said choke coil inseries with the choke coil between said high frequency amplifiers, meansconnecting said series circuit in shunt to said direct current source,two pairs of electron discharge modulator systems each having electrodesbetween which impedance exists, means connecting the impedances of apair of said electron discharge systems in series, means connecting thejunction point between said last named series-connected impedances to apoint on said choke coil, means connecting the said impedances of theother pair of said electron discharge systems in series, meansconnecting a point between the said last named series impedances toanother point on said choke coil and means for modulating substantiallyin phase opposition the impedances of pairs of said systems atmodulation potential frequency to modulate said high frequency stages onpositive and negative half cycles of the modulation frequency.

2. A system as recited in claim 1 wherein said last named means includesmeans for modulating the impedances of a pair of said systems through arange greater than the range through which the impedances of anotherpair of said systems is modulated.

3. In a modulation system, two class C high frequency amplifier stages,a source of direct current potential, a choke coil having a pointthereon vconnected'to one of said 'high frequency stages and a secondpoint thei'eon connectedto another.

of said high frequency stages to form with said high frequency stages aseries circuit, means connecting said series circuit in shunt to saidsource of direct current potential, two pairs of electron dischargemodulator tubes each having input electrodes and output impedances,means connecting one of the tubes of a pair of said tubes in shunt toone of said high frequency stages and the other of the tubes of said onepair of tubes in shunt to the other of said high frequency stages andsaid choke coil, means connecting one of the tubes of the other pair ofsaid tubes in shunt to one of said high frequency stages, meansconnecting the other of said tubes of said other pair of tubes in shuntto the other of said high frequency stage and said choke coil, and meansfor impressing modulating potentials substantially in phase oppositionon the input electrodes of said pairs of tubes.

4. A system as recited in claim 3 wherein said last named means includesmeans for impressing modulating potentials on a tube of each pair oftubes of greater amplitude than the modulating potentials impressed onthe other tube of each pair of tubes.

5. In a modulation system, two class C high frequency amplifier stages,a source of direct current potential, a choke coil having a pointthereon connected to one of said high frequency stages and a secondpoint thereon connected to another of said high frequency stages to formwith said high frequency stages a series circuit, means connecting saidseries circuit in shunt to said source of direct current potential, fourelectron discharge modulator tubes each having input electrodes andoutput impedances, an impedance connecting two of said electrondischarge modulator tubes input electrodes in pushpull relation, meansconnecting points on said last named impedance to the input electrodesof the other two of said tubes to connect the input electrodes thereofin push-pull relation, means connecting one of said first two tubes inshunt to one of said high frequency stages and the other of said firsttwo tubes in shunt to said one of said high frequency stages and saidchoke coil, means connecting one of said other two tubes in shunt to theother of said high frequency stages and the other of said other twotubes in shunt to said last named high frequency stage and said chokecoil and means for impressing modulating potentials on said impedanceconnecting the input electrodes of two of said tubes in push pullrelation.

6. A system as recited in claim 5 wherein said four tubes have theirinput electrodes biassed for class B operation.

7. In a modulation system, two class 0 high frequency amplifier stages,a source of direct current potential, a choke coil having a pointthereon connected to one of said high frequency stages and a secondpoint thereon connected to another of said high frequency stages to formwith said high frequency stages a series circuit, means connecting saidseries circuit in shunt to said source of direct current potential, twopairs of electron discharge modulator tubes each having input electrodesand output impedances, an impedance connecting two of the electrondischarge modulator tubes input electrodes in pushpull relation, meansconnecting points on said choke coil to the input electrodes of theother two of said modulator tubes, means connecting one of said firsttwo of said tubes in shunt to one of said high frequency stages and theother of said first two tubes in shunt to said one of said highfrequency stages and said choke coil, means connecting one of said othertwo tubes in shunt to the other of said high frequency stages and theother of said other two tubes in shunt to said last named high frequencystage and said choke coil, and means for impressing modulatingpotentials on said impedance connecting the input electrodes of saidfirst two tubes in push-pull re- 5 lation.

NEWSOME HENRY CLOUGH.

